Only S-norcoclaurine and S-4'-demethylnorreticuline are converted with SAM in the presence of COMT into 0-methylated congeners of the reticuline biosynthetic pathway. The corresponding R-enantiomers are methylated at different hydroxy groups. This adds more evidence to the theory that mammalian morphine may derive from the same precursor isoquinolines, (S)-norcoclaurine and (S)-reticuline, utilized by the poppy plant and by using the same enzymes.